Regarding separation of a mixture, there are various techniques for removing substances (for example, salts) dissolved in a solvent (for example, water), and membrane separation processes have been used as a process for energy saving and resource saving. Examples of membranes used in membrane separation processes include, for example, a microfiltration membrane, an ultrafiltration membrane, and a reverse osmosis membrane, and in recent years, membranes that fall between the reverse osmosis membrane and the ultrafiltration membrane (loose RO membrane or NF membrane: nanofiltration membrane) have been developed and used. These membranes allow obtaining drinking water, for example, from sea water, brackish water, and water containing harmful substances, and, in addition, have been used, for example, in the production of industrial ultrapure water, wastewater treatment, and recovery of valuables.
Most of the composite semipermeable membranes that are commercially available at present fall within two types: one having on a porous support membrane a gel layer and an active layer in which polymers are cross-linked; and the other having on a porous support membrane an active layer produced by polycondensation of monomers. Above all, a composite semipermeable membrane obtained by coating a porous support membrane with an ultrathin membrane layer composed of cross-linked polyamide obtained by polycondensation reaction of polyfunctional amines with polyfunctional acid halides has been widely used as a reverse osmosis membrane having high permeability and selective separation properties.
However, it is known that a long-time contact of a reverse osmosis membrane with an oxidizing agent such as hydrogen peroxide, hypochlorous acid, or ozone, which is added for decomposition and removal of a solute, sterilization, or membrane washing, or with organic matter such as ethanol and isopropyl alcohol, degrades the membrane performance with time, and improvement in durability has been demanded.